Possible goals in vehicle development are eco-friendly policies and fuel efficiency improvements due to worldwide high oil prices and emission control goals. Accordingly, vehicle makers have made much effort to comply with eco-friendly policies and develop technologies for fuel savings and emissions reductions in order to improve fuel efficiency.
Under the aforementioned circumstances, vehicle makers pay much attention and effort to technical developments of hybrid electric vehicles (HEVs) that use an efficient combination of distinct types of power from an engine and a motor for achieving high fuel efficiency. HEVs satisfy the needs of many customers with high fuel efficiency and eco-friendliness.
FIG. 1 schematically illustrates a configuration of a hybrid vehicle.
Referring to FIG. 1, a hybrid vehicle includes an engine 10 and a driving motor 12 arranged in series, an engine clutch 13 disposed between the engine 10 and the driving motor 12 to make the synchronized connection or disconnection of the engine 10 and the driving motor 12, an automatic transmission 14 providing speed and torque conversions from a power source, i.e., the driving motor 12 or both the driving motor 12 and the engine 10 to driving wheels 30, a hybrid starter generator (HSG) 16 that is a type of motor connected to a crank pulley of the engine 10 through a belt 11 to enable power transmission for engine starting and power generation, an inverter 18 for motor control (control of the motor 12 and the HSG 16) and power generation control, and a high-voltage battery 20 chargeably and dischargeably connected to the inverter 18 to supply power to the motors 12 and 16 and electrical components of the vehicle.
The aforementioned hybrid vehicle is called a transmission mounted electric device (TIED) in which the driving motor 12 is mounted at the automatic transmission 14.
The hybrid vehicle further includes a hybrid control unit (HCU) for controlling the overall operations of the hybrid vehicle, and a battery control unit (BCU) for managing and controlling the battery 20. The BCU is also called a battery management system (BMS).
The hybrid vehicle may provide driving modes, such as an electric vehicle (EV) mode which is a pure electric vehicle mode using only the power of the driving motor 12, a hybrid electric vehicle (HEV) mode which uses the driving motor 12 as an auxiliary power source while using the engine 10 as a main power source, and a regenerative braking (RB) mode which recovers braking and inertial energy of the vehicle through generation from the driving motor 12 during braking operations or inertia of the vehicle and charges the battery 20 with the recovered energy.
In the HEV mode, the vehicle is run by the sum of the output torque of the engine 10 and the output torque of the driving motor 12 through the lock-up of the engine clutch 13. In the EV mode, the vehicle is only run by the output torque of the driving motor 12 through the opening of the engine clutch 13.
Meanwhile, a conventional hybrid vehicle performs power generation by driving an HSG using the power of an engine to charge a battery, when the conventional hybrid vehicle is running in EV mode and the state of charge (SOC) of the battery is less than or equal to a predetermined target SOC. In other words, in an idle state of the engine that is not connected to a driving motor in the synchronized manner, electrical load for power generation is applied to the HSG connected to the engine to charge the battery. Hereinafter, for convenience of explanation, this power generation method is referred to as “idle generation”.
In the idle generation, however, the engine is driven at low rpm (revolutions per minute) and low torque, and in general, the capacity of the HSG is smaller than that of the driving motor. Therefore, the conventional hybrid vehicle may suffer from reductions in fuel efficiency of the engine and power generation efficiency due to the idle generation.